RU130094U1 - ELECTRO-OPTICAL MODULATOR OF POLARIZED RADIATION - Google Patents

Abstract

An electro-optical modulator of polarized radiation, including a crystal with a longitudinal electro-optical effect and transparent conductive coatings of indium and tin oxide, deposited directly on the working surfaces of the crystal, characterized in that the modulating voltage to the transparent conductive coatings is applied in the annular zone around the entire perimeter of the coating through contact rings, which are diffusely bonded by indium with conductive crystal coatings.

Description

The utility model relates to optical instrumentation, namely, to elements of polarization optics intended to convert the state of radiation polarization in optical systems, and can be used in polarization measurements, in particular, in solar magnetographs.

Currently, there is an acute need to solve the problem of effective analysis and control of the polarization parameters of solar radiation for measuring magnetic fields both at high frequencies to reduce the effect of atmospheric jitter, and at very low frequencies at which low-noise radiation receivers operate. The photoelectric analyzer of the full state of polarization uses radiation polarization modulators that introduce an alternating phase shift to determine all the Stokes parameters and obtain the full magnetic field vector in solar magnetographs [1, 2].

The aim of the invention is to increase the service life and improve the operational characteristics of the modulator for controlling the polarization of radiation in a wide frequency range for any form of control voltage (sinusoidal or rectangular).

In polarimetric measurements, electro-optical modulators (EOMs) based on the longitudinal electro-optical effect in solid crystals (Pockels effect) are most common, despite the need to apply high voltage to obtain a significant phase difference between the ordinary and extraordinary rays. Typically, the modulator crystal is placed in a cuvette filled with immersion liquid, and the inner side transparent walls of the cuvette coated with a conductive coating form an electrode system [2, 3]. In essence, the Pockels cell is an electric capacitor. The capacity of such a capacitor, its own leakage currents, is largely determined by the design of the cell. The frequency interval of EOM of modern magnetographs, as a rule, lies in the region of 1 kHz. As complex multichannel systems developed, the modulation frequency decreased, and could be from 8 to 0.5 Hz. A Pockels cell based on an electro-optical DCDR crystal (deuterated potassium dihydrogen phosphate) is used to quickly modulate polarization. The control half-wave voltage for these crystals is 3600 volts (at a wavelength of optical radiation of λ 6328 A). However, the use of low-frequency rectangular modulation leads to a “collapse” of the signal fronts and to a decrease in the modulation depth, as a result of the manifestation of the polarizability effect in an electro-optical crystal placed in a cuvette [3]. In fact, in this case, the electro-optical effect is well manifested at an alternating control voltage at frequencies above 50 hertz and substantially less or completely absent at low frequencies (units of hertz) and constant voltage [4].

To expand the frequency range of electro-optical modulators, a transparent conductive medium is adjacent directly to the surfaces of the electro-optical crystal. Known modulators in which voltage is applied to an electro-optical crystal using an electrically conductive liquid placed in separate transparent cuvettes or directly adjacent to the crystal [5]. However, such modulators have a complex structure to provide liquid electrodes and protect crystals from dissolution and are operable in a limited range of operating temperatures. In an electro-optical cell with plasma electrodes [6, 7], the control voltage is applied to the ends (working surfaces) of the KDP electro-optical crystal (potassium dihydrogen phosphate) through a rarefied gas plasma created by an ionizing electric pulse. Plasma transparent to incoming radiation plays the role of conducting electrodes of a flat capacitor, inside which there is an electro-optical crystal.

However, to obtain the plasma required for the effective operation of the plasma electrodes, vacuum equipment, power supplies, a plasma pulse generator for the discharge current, etc. are required. Near the peak current of this discharge, a generator is turned on, which charges the KDR crystal to the required voltage. In fact, this device gives steep fronts of transmitted radiation, can work as a source of individual pulses, but cannot work as a continuous radiation modulator.

The closest technical solution is the polarization modulator [8], in which a transparent electrically conductive medium is deposited on the surface of an electro-optical crystal with a longitudinal electro-optical Pockels effect. The modulator includes an electro-optical crystal with transparent conductive electrodes - coatings of a mixture of indium oxide and tin oxide deposited directly on the working surfaces of the crystal. The crystal is placed in a protective case in immersion between the optical windows. The supply of modulating voltage to the conductive coating on the crystal surfaces is carried out locally through a high-voltage output.

However, this solution has disadvantages. Despite the fact that in optical modulators the load is purely capacitive, the magnitude of the supplied currents can reach a significant value due to the steepness of the fronts, which they strive to make as high as possible. Therefore, the local connection of high-voltage bushings to the conductive coating causes the destruction of the conductive transparent coating due to the significant current density at the contact point. In addition, when using conductive adhesives to fix coated bushings, the water-soluble DCDR crystal is destroyed by moisture adsorbed by the adhesive. These factors significantly reduce the operational characteristics and the lifespan of the modulator.

In the proposed design, the high-voltage input is carried out through contact rings around the entire perimeter of the conductive coating of the working surface of the electro-optical crystal outside the working area. As a conductive fastening of the ring to the coating, diffusion bonding using indium metal is used, which has good adhesion to most materials and high ductility. The latter quality is important, since the crystal vibrates when an alternating voltage is applied due to the effect of electrostriction. The contact ring itself on the crystal side has the same conductive coating as the electro-optical crystal.

A schematic representation (assembly) of the electro-optical modulator in the frame is shown in Figure 1.

The electro-optical crystal 1 has conductive transparent coatings 2 on two working surfaces. The control signal is connected to the modulator using terminals 10, and then voltage is applied to the conductive transparent coatings of both surfaces of the electro-optical crystal along the entire perimeter through electrically conductive contact rings 3, which have such same conductive coating 4. The contact rings are attached to the conductive surfaces of the electro-optical crystal using an annular gasket from indium 5 due to diffusion of indium into both the contact rings and the crystal when connected under pressure. The assembly, an electro-optical crystal with slip rings, is placed in a frame 6. The working surfaces of the electro-optical crystal are protected from the external atmosphere by optical windows 7 located on immersion 8 inside the slip rings, and the side surface of the electro-optical crystal and slip rings are protected by a sealant 9 and a frame 6.

During the tests, the long-term operability of the device in the mode of constant and variable control signals was proved, while the electric strength of the structure allows reaching half-wave voltages in the visible region of the spectrum. Modulators of polarized radiation have successfully passed tests, having worked for 2 years at the Sayan and Baikal solar observatories.

List of sources used

1. D.Yu. Kolobov, NI Kobanov, V. M. Grigoriev // Instruments and experimental technique. - 2008. No. 1, S.136-141.

2. E.R. Mustel, V.N. Parygin. Methods of modulation and scanning of light M .: Nauka, 1970. - 296 p.

3. V.M. Grigoriev, N.I. Kobanov. Phisica Solari-Terrestris. 1980. Vol. 14. P.77-80. Potsdam. DDR

4. V.S. Markov, G.N. Domyshev, V.I. Skomorovsky. Work electro-optical modulator magnetograph at low frequencies. II. Instability of the current voltage in the electro-optical crystal. Research on geomagnetism, aeronomy, and solar physics, issue 83, pp. 141-149. M .: Science, 1988.

5. M.M.Volynkin, V.A. Malinov, N.V. Nikitin, A.D. Starikov, A.V. Charukhchev. Electro-optical shutter of a large aperture with liquid electrodes. O.M.P. 1986, No. 1, pp. 10-11.

6. M.A. Rhodes, C.D. Boley, A.G. Tarditi, B.s.Bauer, Plasma Electrode Pockels Cell for ICF Lasers, SPIE Proc, vol. 2633, 1997, p. 94-104

7. N. Andreev, A. Babain, V. Bredikhin, V. Ershov. Production of large-sized optics from water-soluble crystals. Photonics, 2007, No. 5, 34-37.

8. Borodin A.N., Petrov A.S., Domyshev G.N., Skomorovsky V.I. "Electro-optical phase modulator", Patent RU 2248601 G02F 1/03

Claims (1)

An electro-optical modulator of polarized radiation, including a crystal with a longitudinal electro-optical effect and transparent conductive coatings of indium and tin oxide, deposited directly on the working surfaces of the crystal, characterized in that the modulating voltage to the transparent conductive coatings is applied in the annular zone around the entire perimeter of the coating through contact rings, which are diffusely bonded by indium with conductive crystal coatings.

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